Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may be intermittently diagnosed for the presence of undesired emissions that could release fuel vapors to the atmosphere. Undesired evaporative emissions may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system and/or an emissions control system may be isolated at an engine-off event. The pressure in such a fuel system and/or an emissions control system will increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and undesired emissions identified based on expected vacuum development or expected rates of vacuum development. However, the entry conditions and thresholds for a typical EONV test may be based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, miles driven, etc. If these conditions are not met, the entry into the evaporative emissions test is aborted. Thus, hybrid electric vehicles, including plug-in hybrid electric vehicles (HEV's or PHEV's), particularly pose a problem for effectively controlling evaporative emissions. For example, primary power in a hybrid vehicle may be provided by the electric motor, resulting in an operating profile in which the engine is run only for short periods. As such, adequate heat rejection to the fuel tank may not be available for EONV diagnostics.
An alternative to relying on inferred sufficient heat rejection for entry into an EONV diagnostic test is to instead actively pressurize or evacuate the fuel system and/or emissions control system via an external source. For example, a method may perform a pressure-based evaporative emissions test using a pump to pressurize and/or evacuate the fuel system and/or emissions control system. The fuel system and/or evaporative emissions control system may then be monitored for a selected time period, and if the pressure falls below a threshold value if initially pressurized, or rises above a threshold value if initially evacuated, the system identifies undesired emissions. As such, by conducting evaporative emissions tests via the use of an external pressure source, reliance on heat rejected from the engine may be circumvented.
In one example, the external pressure source may comprise engine intake manifold vacuum during engine operation. In such an example, the fuel system and/or evaporative emissions system may be sealed from atmosphere, and subsequently engine intake manifold vacuum may be applied to the fuel system and evaporative emissions system by commanding open a valve positioned between the fuel system and/or evaporative emissions system, and engine intake. With engine intake manifold vacuum applied to the fuel system and/or evaporative emissions system, pressure in the fuel system and/or evaporative emissions system may decrease to a predetermined negative pressure threshold. Once the predetermined negative pressure threshold is reached, the fuel system and/or evaporative emissions system may be sealed from the engine, and pressure bleed-up monitored. An increase in pressure to a threshold pressure level during a predetermined time duration may indicate undesired evaporative emissions. However, in such an approach, during the pressure bleed-up phase, fuel slosh from road feedback may skew results as a result of increased pressure in the fuel system due to fuel movement. Some examples where fuel slosh may be an issue for an evaporative emissions test relying on pressure bleed-up may include situations where a vehicle operator or passenger enters a car and/or moves around in a seat, when a door is slammed, when a truck is opened and/or closed, when the vehicle is driven in a stop and go fashion, or when the vehicle is driven on windy and/or bumpy roads. If slosh is detected, via a fuel level sensor for example, the evaporative emissions test may be aborted, thus decreasing completion rates for evaporative emissions test diagnostics. Federal emission regulations require completion rates above preselected rates.
U.S. Pat. No. 6,308,119 teaches diagnosing undesired evaporative emissions at engine idle, where the evaporative emissions system is closed and drawn down to a reference negative pressure during a drive cycle via engine intake vacuum. Upon an indication that engine idle is achieved, the evaporative emissions system is sealed from the engine intake vacuum, and the evaporative emissions test conducted by monitoring bleed-up as described above. However, the inventors herein have recognized potential issues with such a method. For example, in such a method, fuel in the fuel system may be hot, and may thus contribute to increased pressure in the fuel system and evaporative emissions system during the evaporative emissions test procedure, potentially resulting in false failures. Additionally, while U.S. Pat. No. 6,308,119 teaches sealing the evaporative emissions system from the engine responsive to an indication that the vehicle is at engine idle, the act of stopping the vehicle may result in waves in the fuel that may translate into fuel vaporization, thus raising pressure in the fuel system and evaporative emissions system and potentially resulting in false evaporative emission leaks.
Thus, the inventors herein have recognized the above issues, and developed systems and methods to at least partially address them. In one example, a method is provided, comprising responsive to a command from a location external to the vehicle to start a combustion engine in the vehicle, reducing pressure in a fuel system which supplies fuel to the engine and an evaporative emissions system coupled to the fuel system to a predetermined pressure, and indicating undesired evaporative emissions responsive to a subsequent pressure increase rate in the fuel system and evaporative emissions system greater than a threshold pressure rate.
In one example, applying negative pressure on the evaporative emissions space to conduct a test for undesired evaporative emissions further includes indicating that doors and a trunk of the vehicle are closed, and that fuel temperature in a fuel tank which supplies fuel to the engine is below a fuel temperature threshold, wherein the fuel temperature threshold comprises fuel temperature where fuel does not readily vaporize. In this way, by conducting a test for undesired evaporative emissions during conditions wherein the vehicle engine has been started remotely, and wherein the vehicle is indicated to be unoccupied, with doors and trunk closed, and without fuel vaporization, conditions that may skew results of such a test may be avoided, and completion rates increased.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.